1. Field of the Invention
The invention relates to the wireless delivery of energy, and in particular to a wireless power transfer system that allows energy, such as RF energy, to be transferred wirelessly to moving targets using nonradioactive resonant coupling.
2. Description of the Related Art
In recent years, rapid technological advances in microelectronics, nanotechnology and MEMS technology have spurred new types of implanted devices. Implantable devices have become increasingly popular in modern medicine. These devices have a wide range of applications, such as health monitoring, disease prevention, delivery of a therapeutic regimen, and biomimetic prosthesis. For example, the deep brain stimulation (DBS) device is used as a brain implant for treating Parkinson's disease and tremor; electrical stimulation of nerve tissue and recording of neural electrical activity are the basis of emerging prostheses and treatments for spinal cord injury, stroke, sensory deficits, and neurological disorders. However, supplying electrical power to implants within a biological body (e.g., a human or an animal) has remained a significant problem.
Despite improvements in energy density of implantable batteries, periodic surgeries are still required to replace a depleted battery, which involves high cost and surgical risks. For example, the expected life of implanted devices for the treatment of epilepsy produced by Inc. Cyberonics, U.S.A., is no more than 8 years due to the limited battery capacity; the battery in deep brain stimulators manufactured by Inc. Medtronic, U.S.A., require surgical intervention to replace the entire device after depletion. The total charge for this surgical procedure is approximately $25,000, which has been described as “the world's most expensive battery change”. On the other hand, for artificial hearts with high power requirement, energy is commonly provided by external batteries through percutaneous wires susceptible to infection and a limited range of movement. In cases where rechargeable batteries are used to support implants, successful application is currently limited as a result of various issues related to the size, weight, longevity, toxicity and safety of batteries.
Wireless power transfer (WPT) is a promising technology for a variety of implantable devices. This technology reduces not only the risk of infection due to cables passing through the skin, but also the size of the device so that powering an implant no longer depends on the battery. Up to the present, based on magnetic coupling, the amount of wireless power that can be delivered transcutaneously to the animal in a reliable fashion has been too small for most stimulation implants, and the feasibility of performing WPT while allowing free motion of the laboratory animal within a relatively large experimental site has remained unproven.
Wireless electricity represents a new WPT technique based on strongly coupled resonance via evanescent fields in the midrange of coil separation. The resonant interaction between the transmitter and receiver can be strongly established, allowing a relatively high WPT efficiency even when misalignment arises due to operating conditions in a real-world scenario. Moreover, a resonance-based WPT system can reduce energy dissipation into biological tissues because such tissues are off-resonant at the operating frequency.
A typical wireless energy transfer system includes wearable transmitter coils to transcutaneously power implantable devices. The system is also used to power a wireless sensor network embedded within the clothes in the medical and military fields. However, in order for wireless electricity to be used for powering implanted medical devices, the user must always carry a radio frequency amplifier and wear transmitter coils, posing major restrictions on his/her free movement. This is inconvenient as well as unsuitable for experimental animals.
In other consumer and industrial applications, a wireless charging platform based on wireless power transfer can recharge electronic devices such as laptop, cell phone, and media player, etc. Moreover, wireless power transfer can used to deliver the energy from a solar panel outside a residential house to the inside without a cable through the construction wall or roof. The applications of wireless power transfer also extend to powering a robot or an electric vehicle (EV) by a series of thin energy cells under the floor or paved road, recharging EVs by an unobtrusive wireless charger “mat” on the garage floor, or transferring solar energy to the inside of a parking car to power ventilation fans in order to keep the inside temperature from rising too high.